1. The Invention
This invention relates to an improved process for manufacturing nitric acid. In a further aspect, the invention relates to an improved process for preparing nitric acid comprising an improved method of disposing of the nitrogen by-products. In a still further aspect, the invention relates to a improved nitric acid process wherein the amount of treated fresh feed water required is substantially reduced. In another aspect, the invention relates to methods of disposing of liquids containing ammonium nitrites.
2. Prior Art
As is well known, nitric acid is generally prepared by the catalytic oxidation of ammonia with air to yield nitrogen dioxide, which in turn is reacted with water, typically in an absorption column, to yield aqueous nitric acid. Note, for example, page 320 of the test "General Chemistry for Colleges", Smith, Hopkins and Bailar, edit. Heath & Co. (1951), which discusses the primary reactions involved in the basic process, and U.S. Pat. No. 2,697,652, which sets forth in industrial process.
One of the major problems associated with the manufacture of nitric acid is the disposal of waste gas streams and/or waste liquid streams containing various compounds of nitrogen. These compounds are quite undesirable from an environmental standpoint. The gas pollution problem is typically controlled by catalytic abatement or by passing the gases issuing from the nitric acid absorption column through a water scrubber. The catalytic abatement method is necessarily expensive to operate. In the scrubber system, the nitrogen oxides content of the gas is reduced to an acceptable level for disposal to the atmosphere. However, in scrubbing the gas stream, a polluted liquid stream is created. The disposal of this liquid stream issuing from the scrubber is not a simple problem, because the liquid stream contains too high a content of nitrogen compounds (e.g., ammonium nitrate, ammonium nitrate, nitric acid) to permit disposing of it directly to municipal sewer systems or natural bodies of water. Thus, either further processing is required to reduce the nitrogen compound content to an acceptable level, or an alternative use must be made of this liquid. The disposal problem is further compounded by the presence of ammonium nitrite, which presents a hazard because of its instability.
U.S. Pat. No. 3,579,298 teaches, with respect to a combined nitric acid-ammonium nitrate process that one would expect that ammonia-contaminated quench waters could be used in the nitric acid absorption step, but then cautions that if the ammonia present in this quench water reacts with nitric oxide to form ammonium nitrite, "which will not react with the nitric acid but which is a very unstable compound, decomposing in hot water well below 212.degree. F., which ammonium nitrite will move up or down the absorption column, recomposing and decomposing and depositing in pipes or vessels connected to the gas exist of the absorption column, or possibly getting into the final product. Anywhere the ammonium nitrite deposits, it will be intolerable because it is liable to spontaneous decombustion, which may well be hazardous." See Col. 1, line 62-Col. 2, line 7.
In German Patent No. 2,513,619 an improved scrubber system is disclosed for treating the tail gases issuing from the nitric acid absorber. In this treatment, the tail gases are treated with ammonia and water in a scrubber column, yielding an aqueous stream comprising nitric acid, ammonium nitrate and ammonium nitrite. Thus the liquid effluent from this process differs from that from the ammonium quench water referred to in U.S. Pat. No. 3,579,298 in that it does not contain ammonia, but in fact already contains ammonium nitrite. One method used by the art of handling this liquid has been to accumulate the liquid from the scrubber in a heated holding tank wherein the ammonium nitrite is decomposed under controlled conditions to nitrogen and water and then to feed the liquid to an evaporator wherein the ammonium nitrate content of the liquid is concentrated to a degree wherein it can be used in the fertilizer industry. The problem with this process is that it requires a large expenditure of energy, first to initially heat the liquid to decompose the ammonium nitrite and then a further, larger expenditure of energy to evaporate off a substantial amount of water. Also, prior to evaporation the liquid must be neutralized. For example, in typical operation the liquid from the holding tank is first neutralized with ammonia and then concentrated from a combined ammonium nitrate-nitric acid concentration of about 25% by weight to an ammonium nitrate concentration of around 83%. Since it is uneconomical to recover the large amount of water evaporated from the liquid, this water is generally merely vented to the atmosphere along with some ammonia, resulting in an ammonia loss and a substantial water loss and risking further pollution emissions such as ammonium nitrate particulates or aerosol. Further, the fresh feed water used in the process must be treated to remove deleterious ions (e.g., chloride), or render them innocuous. The water loss also generates increased treatment costs.
Thus, it would be very desirable to provide an improved process which would eliminate these energy expenditures and material losses relative to the disposal of waste nitrogen compounds.
Waste gases comprising oxides of nitrogen are, of course, also generated by other industrial processes, such as, for example, pickling, vat cleaning etc. These gases can be disposed of by catalytic abatement or by scrubbing with ammonia and water. Thus, where the scrubbing method is used, a waste liquid stream containing ammonium nitrate and/or ammonium nitrite is created. Thus, it would be desirable to provide a process for economically disposing of such waste liquids.